Various types of video image enhancement apparatus and techniques have been proposed. Automatic Gain Control (AGC) techniques have been employed, inter alia, in video signals for reducing the dynamic range of the video signal by subtraction of its DC level. Such a technique is described in U.S. Pat. No. 4,719,350, wherein linear AGC is applied to the video image on a pixel by pixel basis. This technique is applicable only to a solid area which is delimited by a line parallel to the scan direction.
Generally speaking, AGC is employed in video processing only on a frame by frame basis.